Sourceforge had integrated mailing-list support, bug trackers, page-hosting, and more.
There was a community there, it was just that each community was based around a particular project. There was little chance of a user of project A from interesting with project B. But I guess the same could be said of github.
Sourceforge failed in part because of feature-creep, and availability issues. But I think it would be unfair to say that it wasn't "social".
Found some tiny project that does almost everything you want, but has a tiny feature you can implement easy? No worries, fork, add it.
Even if it's never pushed upstream, it's easy for someone to find and perhaps use themselves.
I do this with things like zabbix templates/scripts and docker containers that are 99% what I want.
Previously there was no easy way to do this - I'd have to set up an entirely new sourceforge/google code/etc account, which is a lot more effort.
That's the part that wasn't really possible before git though. But you could always do an anonymous checkout and make your changes there,you could even do a pull request (via a patch). There was no need to signup for that, even though most of us had a source forge account (I still do) anyway.